1. Field of Technology
The present invention relates to a platen gap adjustment assembly for adjusting the platen gap in a printing apparatus between a platen and print head for printing on a print medium transported between the platen and print head and to a printing apparatus including a printer for printing to the print medium. The platen gap is the defined distance between the print head and the platen.
2. Description of Related Art
Some printers use a platen to control the position of the print medium to the print head, position the platen to the print head, and print to a print medium inserted between the platen and print head. The thickness of the print medium may vary according to the application. The appropriate distance between the print head and the print medium also depends upon the printing method. The appropriate platen gap is therefore determined by the thickness of the print medium and the appropriate distance between the print head and print medium. If the thickness range of usable print media is limited to a specific, narrow range, the platen gap can also be set to a fixed distance. However, if the platen gap is fixed when the thickness range of the usable print media is somewhat greater, it may not be possible to maintain the appropriate distance between the print head and the print medium. In this situation the platen gap is first adjusted to a constant default setting, and the platen gap is then changed to the appropriate distance either automatically or manually according to the thickness of the print medium being used.
Japanese Unexamined Patent Appl. Pub. H08-25721 teaches a platen gap adjustment device that works by moving the platen, and Japanese Unexamined Patent Appl. Pub. H10-211748 teaches a platen gap adjustment device that works by moving the print head. Both of these devices use a cam and an eccentric shaft that can rotate on a central axis of rotation, and rotate the cam or the eccentric shaft in order to move the support shaft (control shaft, guide rod) that axially and slidably supports the platen or print head. The cam or eccentric shaft is then stopped at the rotational position yielding the appropriate platen gap, and a shaft-engaging part rendered in unison with the cam or eccentric shaft is then engaged with a frame-engaging part rendered on the printer frame, for example.
The platen gap adjustment device requires a relatively strong force to keep the shaft-engaging part engaged with the frame-engaging part sufficiently to withstand the vibrations accompanying print head movement and the reaction of the print head when printing with a dot impact method. An equal force is also required to overcome this engaging force and disengage the shaft-engaging part from the frame-engaging part in order to rotate the cam or eccentric shaft and adjust the platen gap. In order to thus hold and adjust the platen gap, the device taught in Japanese Unexamined Patent Appl. Pub. H08-25721 renders the engaging parts at a position offset from the center shaft. The length of the lever from the center shaft to the engaging part is thus great enough that a small force is sufficient to disengage the engaging parts. By thus positively securing an engaging part separated from the center shaft, the cam or eccentric shaft can be reliably secured against a large force acting circumferentially to the center shaft.
A problem with locating the engaging parts at a position separated from the center shaft is that the size of the adjustment device comprising the engaging part and the cam or eccentric shaft increases. If the size of the adjustment device increases, the size of the printer incorporating the adjustment device also increases.
An object of the present invention is therefore to provide a platen gap adjustment assembly for a printing apparatus that can adjust the platen gap and maintain a constant platen gap without increasing the size of the platen gap adjustment device. A further object of the invention is to provide a printing apparatus comprising this platen gap adjustment assembly.